Avoid reuse in azure pipelines and split build out (Qiskit#260)

* Avoid reuse in azure pipelines job configuration

In the azure pipelines runs for Qiskit#258 the jobs are failing for what looks
like a mismatch with the installed aer python package and what should
have been built. Just to rule that out for runs this commit makes 2
changes. First it creates a conda environment/virtualenv with a unique
name for each run. Secondly it runs git clean to remove any old built
wheels (assuming there are any). Hopefully this will provide more
reliability in future runs.

azure-pipelines.yml

@@ -21,19 +21,25 @@ strategy:
 steps:
   - powershell: Write-Host "##vso[task.prependpath]$env:CONDA\Scripts"
     displayName: Add conda to PATH
-  - script: conda create --yes --quiet --name qiskit-aer
+  - script: conda create --yes --quiet --name qiskit-aer-$(Build.BuildNumber)
     displayName: Create Anaconda environment
   - script: |
-      call activate qiskit-aer
-      conda install --yes --quiet --name qiskit-aer python=%PYTHON_VERSION% numpy
+      call activate qiskit-aer-$(Build.BuildNumber)
+      conda install --yes --quiet --name qiskit-aer-$(Build.BuildNumber) python=%PYTHON_VERSION% numpy
     displayName: Install Anaconda packages
-  - script: |
-      call activate qiskit-aer
+  - bash: |
+      set -e
+      source activate qiskit-aer-$(Build.BuildNumber)
+      git clean -fdX
       python -m pip install --disable-pip-version-check pip==18
       pip install cython
       pip install git+https://github.com/Qiskit/qiskit-terra.git
       pip install --ignore-installed -r requirements-dev.txt
       python setup.py bdist_wheel -- -G "Visual Studio 15 2017 Win64"
-      pip install --find-links=dist\ qiskit_aer
+    displayName: 'Install Dependencies and Build Aer'
+  - bash: |
+      set -e
+      source activate qiskit-aer-$(Build.BuildNumber)
+      pip install dist/qiskit_aer*.whl
       python -m unittest discover -s test/terra -v
-    displayName: 'Install dependencies and Run Tests'
+    displayName: 'Install Aer and Run Tests'